Hemodilution H as been applied traditionally during deep hypothermic circulatory arrest (DHCA) to couteract the increase in viscosity and deleterious rheological efforts which previous reports linked with brain injury. We have developed a unique piglet model of DHCA with magnetic spectroscopy(MRS) and near-infrared spectroscopy (NIRS), then evaluation of neurological status for 4 days and finally histological assessment. The current hemodilution protocol of 3 centers for neonatal DHCA have been investigated using this model. 15 piglets were randomized into 3 groups (n=5/Gp). Gp I= colloid and crystalloid prime, Hct,10% ; Gp II= blood and crystalloid prime, Hct=20 % ; Gp III = blood prime, Hct=30%. All groups underwent 50 min.DHCA at 15 C followed by 45 min. rewarming. High energy phosphate and cerebral intracellular pH were determined by MRS with assessment of cerebral redox state by NIRS. Neurological recovery was evaluated daily for 4 days by neurological deficit score and overall performance categories. Brain was fixed in situ on day 4 and examined by neurohistological score in a blinded fashion. The results are expressed as mean +/- SEM. The NDS was best preserved in Gp III (I=95.8, II=112.5, III=58.0, p<0.05 II vs. III) during first day of recovery, although this difference diminished with time and all animals were neurologically normal after 4 days. Similar trend was observed in OPC score. Histological assessment was worst among Gp I in neocortex area( I=1.33, II=0.22, III=0.40, p<0.05 I vs II, I vs. III). In summary, severe hemodilution (Gp I) results in evidence of inadequate oxygen delivery during early cooling. Moderate dilution (Gp II) supplied adequate oxygen to the brain before and immediately after DHCA. Subsequent deterioration in redox state and neurological score may be secondary to edema resulting from inadequate colloid in the prime. Whole blood priming (Gp III) with higher hematocrit improve cerebral recovery after DHCA relative to moderate and severe hemodilution.